General Information About MGPs

MGP is an abbreviation for Manufactured Gas Plant. A manufactured gas plant was an industrial facility at which gas was produced from coal, oil and other feedstocks. The gas was stored, and then piped to the surrounding area, where it was used for lighting, cooking, and heating homes and businesses. The first MGPs in New York were constructed in the early 1800s, prior to the Civil War. Most were closed during the early-to-middle 1900s, and the last one ceased operations in 1972.

Gas from MGPs was used for all the same purposes that natural gas is used for today. In addition, in the late 1800s, gas was used for lighting prior to the introduction of electricity.

For a period of over 100 years, manufactured gas plants (MGPs) were an important part of life in cities and towns throughout New York State and the United States as a whole. They had their beginnings in the early 1800s, providing small amounts of gas for street lighting systems. By 1900, production had greatly increased, and gas was being widely used for heating and cooking. Most towns in New York with populations of over 5000 had at least one gas plant, and larger towns often had more than one. New York City had several dozen.

Small-town facilities began to close in the 1920s and 1930s as the industry consolidated production at larger facilities and connected smaller systems together with new pipeline networks. As World War II approached, interstate pipelines were built, making natural gas from the Midwest more widely available, and cheaper than manufactured gas. Most New York State MGPs closed by 1950, but a few remained in operation in remote areas, or on standby status in areas where the interstate pipelines could not meet peak demand. The last MGP in New York State ceased operations in 1972.

Two main processes were used to produce the gas. The older and simpler process was coal carbonization. In this process, coal was heated in closed retorts or beehive ovens. Inside these ovens, the coal was kept from burning by limiting its contact with outside air. Volatile constituents of the coal would be driven off as a gas, which was collected, cooled, and purified prior to being piped into the surrounding areas for use. The solid portion of the coal would become a black, granular material called coke. Coke was a valuable fuel for many industrial uses and for home heating, because it burned hotter and more cleanly than ordinary coal. Sometimes, the coke was the primary product, and the gas was a by-product, and the facility was called a coke plant.

As the gas manufacturing industry developed and expanded after the Civil War, a new process was introduced which produced a gas mixture that burned hotter and brighter. This process, carburetted water gas (CWG), was first introduced in the 1870s. By 1900, most MGPs in New York State were using this process. However, some MGPs in the state never made the conversion and continued as coal carbonization facilities for their entire lives.

A variety of water gas processes were developed, all of which involved a first step in which coke or coal was heated in a closed vessel or retort into which steam was injected. A chemical reaction took place which produced a flammable gas mixture of methane and carbon monoxide. Petroleum products were then sprayed into the hot gas mixture, creating another chemical reaction in which petroleum constituents were "cracked" to form methane, which increased the heating and lighting value of the gas.

The production of manufactured gas created wastes, some of which may still remain at former MGP sites.

A dense, oily liquid known as coal tar would condense out of the gas at various stages during its production, purification and distribution. Although most of the tar was collected for sale or reuse, recovery was incomplete. Most plants had tar/water separators, which sometimes could not fully separate the two liquids. The resulting tar/water emulsion was often discharged to a nearby surface water body. Over the decades during which many of these MGPs operated, substantial amounts of tar also leaked from storage and processing facilities and contaminated surface soils, subsurface soils, and groundwater.

Today, at a number of former MGP sites, tar or tar/water emulsions continue to migrate slowly in the subsurface and may enter into sewers, basements, or nearby surface water bodies. Under some conditions, tar will temporarily float on the top of surface water bodies, creating oily sheens on the water surface. However, in most cases, the tar will sink to the bottom, leading to contamination of sediments. Whether present due to historic disposal or continuing migration, coal tar may impact water quality and the organisms which live or feed in the sediment.

Another byproduct, purifier waste, was made up of either lime or wood chips treated with iron oxides, and was used to remove cyanide and sulfur from the manufactured gas. Once it had become saturated with impurities, purifier waste was often discarded or used as a fill material. This waste often contains complexed cyanide compounds which can contaminate groundwater. Purifier waste also generates a strong, objectionable odor when it is exposed on the ground surface. For more information on these MGP wastes, see the section on MGP Wastes.

Our best estimate is that there were roughly 300 sites where manufactured gas was produced either for distribution to the public or other uses. Of these, remedial programs are either under way or scheduled to start at 194. NYSDEC is currently working to identify how many other sites may exist in New York, and where they are located. At this stage, however, it appears that the utility-operated sites currently identified represent the most significant MGPs, by virtue of their larger size.

What are the Current Uses of MGP sites?

Former MGP sites have found a variety of uses in the years since they ceased operations. Many are still owned by the utility companies and are used as electric substations, storage yards, truck garages, office buildings and major generating stations. Many also still contain gas regulating facilities, due to their access to the gas distribution system. Other uses range from abandoned industrial property, to commercial/retail uses, to schools and residences.

These sites often contain abandoned underground structures and pipes containing coal tar or other MGP residuals. Some of these waste materials (especially coal tars) may have migrated from existing/former structures and may be present in the subsurface. Impacts to surface water bodies and their sediments are also common since MGPs were typically located near a source of water.

It is not common to find MGP wastes exposed on the ground surface. Most of these plants have been closed for at least 50 years, and in some cases over 100 years. In many cases, subsequent redevelopment of the MGP sites has removed or covered wastes that were exposed at the surface. However, exposed wastes are sometimes found, and on some sites coal tars may migrate upwards to the ground surface from below.